1. Field of the Invention
This invention relates to a composition for retarding the evaporation of water from dry cooling towers used in power plants and other bodies of water susceptible to evaporation.
Cooling towers of this type reject waste heat from power plants directly to the atmosphere rather than to bodies of water. These towers which function as heat exchangers resemble a large farmer's harrow and consist of steel discs which rotate between hot condenser water from the power plant and the atmosphere. Rotational speeds are about 6 rpm. As the disc rotate and as air is blown over the upper portions of the discs, local sections of the discs are heated in the water and cooled in the air. Thus there is a net flow of heat from the water to the atmosphere. To prevent the discs from carrying a thin film of water up into the air stream and losing water by evaporation (and thereby defeating the purpose of a dry tower), water must be stripped off the discs. The most advantageous way of doing this is to float a layer of oil on the surface of the water. If the oil preferentially wets the disc, water will be stripped off the disc, and air-water interfaces will be eliminated. The normal operating temperatures of the towers range from 100.degree. to 160.degree. F. in the summer and lower in winter, as limited by the ambient air temperature.
2. Description of the Prior Art
Various compositions have been suggested and tried for retarding the evaporation of water from exposed surfaces. Thus in coassigned U.S. Pat. No. 3,459,313, there is disclosed a water evaporation retarding composition consisting of a mineral lubricant oil, trimer or dimer acid, a spreading agent, and a petroleum or natural wax. For particular application in dry cooling towers dimethyl polysiloxane and paraffinic oils, have been tried. There were found to be poor in demulsifiability, air entrainment and metal wetting characteristics.
Experimental work has shown that an evaporation retarding oil composition to be suitable for this application must fulfill certain requirements. The composition must have a specific gravity less than 1.0. It must wet the disc surface and strip water from its surface. Moreover, its cost must be relatively low because of volatilization and other losses.
Additionally, low vapor pressures are required to minimize system losses. Pressures of about 0.1 micron hg are acceptable for normal operating temperatures. This figure becomes significant as the process requires a large surface area (10.sup.7 ft.sup.2). Lower vapor pressure is also desirable from a flammability standpoint.
As concerns the viscosity of the oil, the lower the viscosity the better. A viscosity of 20 cst or lower allows for a fast flowing composition and a thin oil layer on the disc surfaces. A thick oil layer represents a significant thermal resistance to heat transfer, and impairs the disc performance.
With the discs continually cutting the air, water and oil interfaces, and with water flowing between the discs, energy is continually being added to the interfaces, and may generate emulsions or foam. Some oil pumping is necessary, and large water drop entering the pumps may be broken into smaller, more stable drops. Foam or emulsions are undesirable as they will increase water evaporation, increase the chance of oil traveling to the stream condenser, and will generally decrease the periodic tower's performance. Therefore a low foaming oil or an oil with nonfoaming additives is desired. Besides general compatibility, it is desired that the oil (or its additives) provide some corrosion protection for the discs.
Since the condenser water, trough, discs, and oil are potential sites for organic growth, not only must the oil not support growth, but it must be compatible with water treated to prevent organic growth. Since the oil will be exposed to chemicals and gases typical of the power plant environment, it must retain the aforementioned properties sufficiently so that constant replacement is not required.
Exposed to the air, the oil will become impregnated with dirt and dust. Typical oil cleaning mechanisms and techniques must apply to the oil.
There is thus a high degree of criticality in selecting a narrow boiling range oil of the proper viscosity along with the proper additives to furnish the other requirements for the system.